1. Field of the Invention
The present invention relates to an optical scanning system and, in general, relates to an optical scanning system used with various types of optical scanning apparatus for scanning a light beam as a spot image on an image-forming plane separated from the light source, or reversely, for optically reading the graphic image of an object and, more particularly, relates to an optical scanning apparatus in which high-resolution scanning is stably embodied.
2. Description of Related Art
Generally, an optical scanning apparatus installed in a laser printer, a facsimile, etc., consists of a semiconductor laser as a light source, a rotary polygon mirror as a deflector, and a plurality of lenses respectively arranged between the semiconductor laser and the rotary polygon mirror and between the rotary polygon mirror and the image-forming plane (light-sensing drum). The scanned light beam of such optical scanning apparatus is generally described as being divided into a main scanning direction and a subscanning direction. Here, the main scanning direction signifies the direction of a light beam scanned by deflection on a perpendicular plane to any point on a scanning optical axis, and the subscanning direction signifies a direction perpendicular to the main scanning direction. Also, the optical systems of an optical scanning apparatus are described as being divided, according to the scanning direction, into a first optical system in which optical performance is characterized by the main scanning direction, and a second optical system characterized by the subscanning direction. In other words, the first optical system is referred to as the section in which a beam emitted from a light source is shaped in the main scanning direction, and the function of forming the shaped beam onto an image-forming plane is performed after the shaped beam is deflected at a deflector (rotary polygon mirror). The second optical system is referred to as the section in which a beam emitted from a light source is focused onto the deflector in the subscanning direction and the function of forming the deflected beam onto an image-forming plane is performed.
To produce a high-resolution scan in an optical scanning apparatus, a means for controlling the scanning line pitch error which occurs because the mirror surfaces of the polygon mirror deflector are tilted with respect to the subscanning direction when the rotary optical polygon deflector rotates must be taken into consideration. Accordingly, a method for compensating for the tilting of the mirror surface to the subscanning direction is required. In general, an optical system is designed where a light beam is focused on the mirror surface of a deflector in the subscanning direction and is shaped in the main scanning direction, and the deflector and the image-forming plane are in a conjugate relationship in the subscanning direction in the optical system for image-forming.
For example, in U.S. Pat. No. 3,750,189, an optical system for image-forming is composed of a beam shaping means and a focusing means. Namely, a beam emitting and reflecting after focusing on a deflector is shaped, and focused again to form a point-image on an image-forming plane. In the beam shaping means, many lenses are required because the freedom in design for concurrently achieving a uniform scan function and a point-image-forming function is restricted.
In U.S. Pat. No. 3,946,150, a conjugation relationship between a deflector and an image-forming plane is made by arranging a long cylindric lens in the scanning direction between a lens having a uniform scan function and the image-forming plane. In this structure, the lens itself must be quite large because good beam-quality cannot be acquired if the cylinder-type lens is not located near the image-forming plane, and if the scan width is wide, realization of the image forming system itself cannot be attained.
In U.S. Pat. No. 4,379,612, an optical system for image-forming consists of a spherical single lens and a toroidal single lens so as to correct plane-tilting and reflection-point-wandering on a deflection surface. However, the two-lens structure cannot correct beam spread in the main scanning direction, and thus, there is a problem of not being able to achieve a high-resolution scan.
In U.S. Pat. No. 5,136,412, an optical system for high-resolution scanning consists of two spherical lenses, a toroidal lens and an air lens-well adjusted between the two lenses. However, compared with usual optical systems, to obtain a convex air lens that maintains a tight air gap is difficult with an image-forming lens having a large diameter, and thus, this optical system is not appropriate to forming a stable point-image.